Conventional heat-sealing processes usually involve pressing together the surfaces to be sealed followed by the external application of heat. This is conventionally effected by gripping the superimposed layers between a pair of reciprocating heated jaws, or by passing the superimposed layers between a pair of rotating heat-seal bars, or by carrying the superimposed layers between two moving endless metal bands which pass between stationary heated members (a band sealer). In each case, the heat has to pass through the sheet material to reach the contacting surfaces and effect the seal. In the case of thick materials this can result in long dwell times, and even deterioration of the outer surfaces before the inner surfaces are brought to the correct temperature. A further problem is that of ensuring that the contacting surfaces are clean, particularly if the heat-seal is being effected at the mouth of a sack containing powdery material, since it is virtually impossible to ensure that the mouth region of the sack is not contaminated by the powder during filling. To overcome this latter problem a rotary suction device has been proposed for cleaning sack mouths prior to heat-sealing by conventional means.